This invention relates generally to cordless telephones and, more particularly, to a circuit for providing a constant current to an output stage of a cordless telephone transmitter using AC power lines as a transmission medium. Still more particularly, this invention relates to a constant current source for regulating RF signals in an AC power line acting as a transmission medium for cordless telephones to within regulated limits to minimize the effect of impedance variations in those lines while maximizing the RF output signal within those prescribed limits.
In the prior art, cordless telephone systems are known which include a cordless telephone and a base unit, each of which includes a transmitter and a receiver for exchanging modulated RF carrier signals, modulated by telephone switching signals and audio signals to and from the cordless telephone. In general, the cordless telephone includes a speaker, a microphone, an antenna, a transmitter, and a receiver for communicating telephone switching signals and audio signals to and from the base unit via the RF carrier signal. The base unit includes an antenna, a transmitter, and a receiver for similarly communicating the modulated RF carrier signal to the cordless telephone. Such systems generally operate in a duplex mode permitting simultaneous transmission and reception of the signals and require the use of two frequencies for each cordless telephone. Such devices are regulated as low power communication devices by the Federal Trade Commission (FCC). In the past, cordless telephones have operated at a frequency between 26.9 and 27.3 MHz, while currently many cordless telephone systems operate at about 1.7 MHz. A prior art cordless telephone utilized carrier current techniques for the base station transmitter of the phone and a 27 MHz return frequency for the portable handset transmitters. As is well known, carrier current systems use a technique which couples radio frequency (RF) energy into the electrical wiring or the telephone wiring which act effectively as an antenna for reception by a nearby receiver. Thus, the telephone and electrical wiring of the site of the base unit become a cable acting as an antenna. Currently, most carrier-type cordless telephones operate at about 1.7 MHz, while proposals are pending to shift the transmitter frequency of the base station portion of the telephones from 27 MHz to 1.7 MHz or to other frequency ranges using carrier current techniques.
Accordingly, current cordless telephones operate in the duplex mode at 49 MHz for the portable transmitter and 1.7 MHz for the base station transmitter using carrier current techniques. Since carrier current systems are classified by the FCC as restricted radiation devices, the level of RF energy from any part of the system may not exceed a prescribed limit and may not interfere with other authorized radio communications. To this end, the carrier current portion of the cordless telephone is presently restricted to operation at a frequency at a band of 1625 KHz to 1800 KHz wherein the RF currents on the power cord and telephone line of each cordless phone shall not exceed 90 milliamps on any single power conductor or 12 milliamps on the telephone line in accordance with prescribed procedures.
Because cordless telephones are thus limited in their range of operation to a range of several hundred feet from the base station, it is thus a problem in this art to maximize the transmitted RF signal to 90 milliamps in order to maximize the power transmission of the base unit and the associated range of the cordless unit while complying with regulated power transmission limitations.
Maintaining the transmitted RF signal to 90 milliamps has presented an additional problem in the development of cordless telephones because of variations in the wiring of the site in which the base unit is used. Building codes often require that buildings be wired with conductors joined with a ground wire. As a result, the capacitance between the conductors and to ground is significantly high, thus causing considerable attenuation of the high frequency signals involved, which are typically on the order of 1.7 MHz. In addition, building codes also require that in commercial and multiple dwelling buildings, the wiring be located in a conduit which further increases the capacitance since the conduit is grounded. Moreover, inductive loads, such as motors, power transformers, and the like connected to the building or dwelling are effectively in parallel with the capacitance of the wiring and thus form effective high frequency filters, thus affecting the transmission of the RF signal in a cordless telephone.
Accordingly, it is a primary object of this invention to provide a means in a cordless telephone system for minimizing the effects of impedance variations in the AC power lines used as a transmission medium for RF signals, while maximizing the available RF power output from the system to within regulated limits.
It is another object of this invention to provide a circuit which maximizes the power transmission capability of a cordless telephone while limiting the available RF current signal in the AC power lines acting as a transmission current carrier to the prescribed maximum of 90 mA.
It is a further object of this invention to minimize the effects of variations in the power line impedances from one location to another when acting as the transmission medium for the signals of a cordless telephone, while maintaining the RF current on such lines at about its maximum permitted regulated level.
These and other objects of this invention will become apparent from a written description of the invention which follows, taken in conjunction with the accompanying drawings.